Artificial Magnetic Conductors (AMC) are theoretical materials that reflect electromagnetic radiation with zero degree phase change as opposed to Perfect Electric Conductors (PEC) that reflects electromagnetic radiation with a 180 degree phase of reflection (polarity flip) as described in reference (1), which is hereby incorporated in its entirety herein by reference. A key benefit of AMCs is that antenna elements can be placed very close to the AMC surface without the surface shorting out the antenna element as is the case with PEC surfaces. This permits the instantiation of very thin antennas which is a very desirable trait for many antenna applications. Additionally, the AMC will reflect the back lobe pattern of the antenna into the forward direction with no phase inversion: i.e. in-phase and coherent with the front lobe. This produces a 3 dB increase in gain without narrowing the beam (i.e. without changing the Directivity) as the energy that would have gone out the back direction is redirected in phase and with the same pattern as the energy directed in the forward direction.
A central limitation of all AMCs demonstrated to date is narrow fractional bandwidth (typically less than 10% and often less than a couple of percent bandwidth), and a progressive difficulty in achieving any practical useful bandwidth at lower frequencies (a couple of GHz or lower). For those applications that only require a narrow band, or a tuned narrow band, the traditional AMC solutions are adequate. However, many applications and progressively newer applications require wider bands and more bands, effectively requiring Ultra-Wide Band (UWB) performance. By producing a new AMC that has Ultra-Wide Band (UWB) response (defined by DARPA as a fractional bandwidth greater than 25%) a UWB antenna can then be placed in front of and almost in contact with the AMC. The expected result is the instantiation of a true UWB very thin conformal antenna and associated arrays. Anticipated Benefits/Potential Commercial Applications of the proposed design are much thinner antennas and arrays that also cover a UWB bandwidth. This can result in a fewer number of antennas needed in a given application and the ability to build the antenna conformally on or into any surface because of the improved thinness.
Common physical instantiations that manifest AMCs-like properties include Sievenpiper's AMC patches as described in references (2, 3, 4, 5), which are incorporated in their entirety herein by reference, corrugated surfaces and a simple quarter wave standoff between the antenna element and a PEC backplane. The conventional theory of operation of AMCs is discussed in the above-cited references. The key-enabling ingredient in AMC operation is changing the reflection boundary condition. Changing the reflection boundary condition from the PEC boundary condition to a new boundary condition of intentional design produces the desired zero phase reflection response behavior. This phenomenon does not generally happen in naturally occurring materials, and so the required approach must invoke the use of the new field of Metamaterials (1).
Sievenpiper AMCs traditionally are composed of square or hexagonal mushroom-like structures on thin Printed Circuit Board (PCB) substrates. There is usually an implicit, if not always acknowledged antenna like response inherent in their operation. That is, an incident field is presumed to be sufficiently impedance matched to the AMC, such that the electromagnetic wave can be absorbed into the AMC structure. Once the electromagnetic wave has been converted to current in the AMC circuit, the AMC circuit can operate as intended to produce a return with zero net phase. However, as one moves off the resonant frequency of these traditional AMC structures, the AMC system is no longer tuned to receive the incident electromagnetic field. It is perhaps simplistic but easy to illustrate that since many AMC structures resemble and have operational similarities to microstrip patch antennas, that since patch antennas are narrow band, so too must be these AMC structures. Hence its really their antenna receiving properties, and not the underlying AMC circuitry that limits the bandwidth of AMCs. The AMC must convert the incident radiation into current before any AMC behavior can be subsequently produced.
This antenna-like response is at least slightly different than that ascribed to it in the conventional AMC theory, and as one modifies the mushroom shape, the specifics of its response to an incident electromagnetic wave takes on specific behavior that can only be modeled in true electromagnetic wave simulation codes, thereby confirming that the antenna performance aspects of the AMC design limit its bandwidth. If one tries only to modify the AMC circuit for larger AMC bandwidth, this results in larger inductance in the AMC circuit which then results in a larger impedance mismatch with the incident electromagnetic wave, thereby preventing its coupling into the AMC circuit. In effect, the Sievenpiper theory breaks down with significant deviations from the normal design, and the specifics of the implementation begin to matter more and more with such deviations.
For example, in the extreme where the AMC patch is replaced with a wire and large discrete inductors (in order to maximize bandwidth according to the conventional theory), the electromagnetic wave may not couple to the circuit hardly at all, thereby “blowing by” the AMC circuit and hence negating any possible AMC effect. Excessively large inductors can have a similar effect by electrically breaking the AMC circuit at higher frequencies, and excessively large capacitors have the opposite effect, negating effective AMC behavior at lower frequencies. The issue then is that the theory claims such reactance extremes are needed to achieve wide bandwidth operation, but such extremes of capacitance and inductance decouple the AMC circuit from the incident electromagnetic wave, so no net AMC behavior is obtained under these wider band conditions.
The summation of these observations is the somewhat unacknowledged requirement that any AMC must first act enough like an antenna so that it captures the electromagnetic energy from the electromagnetic wave of interest. Only after the electromagnetic energy capture can the resulting current be modified inside the AMC circuit. If this conversion does not happen, then there is no current in the AMC circuit, and the capacitance and the inductance of the AMC circuit can have none of its intended effect to produce a zero phase AMC reflector. At the most fundamental level, this is what prevents the realization of a wide band or UWB AMC.